Vinylidene fluoride (hereinafter referred to as “PVDF”) based (PVDF type) monofilaments have various excellent characteristics and have preferably been in use for fishing lines, such as guts specifically typified by leaders, from the following viewpoint in particular:
Namely, while being excellent in toughness, shock resistance, abrasion resistance, sensitivity (bite detectability), light resistance, and the like, PVDF-based monofilaments have a high specific gravity (1.79), so that they are easy to sink in water. Also, they have a refractive index (1.42) close to that of water (1.33), so that they exhibit less surface reflection in water, whereby they appear transparent and are hard to see. Further, they hardly absorb water and thus can maintain their various characteristics for a long period of time even in water.
They are in use not only for guts, but also for lure lines, main lines for cast fishing, trunk lines for boat fishing, and the like for their high shock resistance, high specific gravity, high sensitivity, and the like.
However, because of their high crystallinity and elasticity modulus, the PVDF-based monofilaments are inherently more likely to kink when wound about a reel as compared with nylon which is generally in use as a fishing line.
It has also been noted that PVDF-based monofilaments are likely to uncoil from a reel when used as a fishing line having a large diameter. They are hard to handle in this case. Therefore, PVDF-based monofilaments further excellent in suitability for reeling have been in demand for lure lines and main lines in particular.
For eliminating such disadvantages, various methods have been tried in order to soften PVDF-based monofilaments. Known as specific examples are:    (1) methods adding a large amount of acrylic resins and polyester type plasticizers (see, for example, commonly assigned Japanese Patent Application Laid-Open No. SHO 54-106622, Japanese Examined Patent Application Publication No. SHO 60-17845 (Japanese Patent No. 1297193), Japanese Patent No. 2566871, and the like);    (2) methods using copolymers (see, for example, Japanese Examined Patent Application Publication No. HEI 5-33059, commonly assigned Japanese Patent No. 2571538, and the like);    (3) methods lowering the draw ratio (see, for example, commonly assigned Japanese Patent Application Laid-Open No. HEI 10-298825);    (4) methods with heavy relaxation heat treatment; and the like.